https://doi.org/10.1351/goldbook.12226
Ratio of \(M_{\rm{m}}\) to \(M_{\rm{n}}\), or the ratio of \(X_{\rm{m}}\) to \(X_{\rm{n}}\) for a homopolymer or an alternating copolymer of sufficiently large molar mass, such that the effects of the distinct structures of the end-groups of the constituent macromolecules can be neglected, giving \(X_{\rm{n}}\) directly proportional to \(M_{\rm{n}}\), \(X_{\rm{m}}\) directly proportional to \(M_{\rm{m}}\) and \(Ð\!_{\rm{M}} = Ð\!_{\rm{X}} = Ð\).
Notes:
- Dispersity is a measure of the dispersion (or spread) of a molar mass, molecular weight, relative molar mass, relative molecular mass, or degree-of-polymerisation distribution function. For a uniform polymer, \(Ð = 1\); for a polymer of sufficiently high \(X_{\rm{n}}\) having a Poisson distribution of molar masses, molecular weights, relative-molar-masses, or relative-molecular-masses, \(Ð \cong 1\); and for a polymer of sufficiently high \(X_{\rm{n}}\) having a most probable distribution of molar masses, molecular weights, relative-molar-masses, or relative-molecular-masses, \(Ð \cong 2\).
- For a copolymer that is not an alternating copolymer \(X_{\rm{n}}\) cannot be considered to be directly proportional to \(M_{\rm{n}}\), nor \(X_{\rm{w}}\) directly proportional to \(M_{\rm{w}}\). It is then necessary to state whether \(Ð\!_{\rm{M}}\) or \(Ð\!_{\rm{X}}\) is being used.